def __init__(self, file_: BufferedRandom, length: int) -> None:
# Read the file into a buffer.
file_.seek(0, SEEK_END)
buf = bytearray(file_.tell())
file_.seek(0, SEEK_SET)
file_.readinto(buf)
# Overwrite the header, in case it is currently being written to.
header = list_header(length)
buf[:len(header)] = header
# Parse the buffer's contents as list items.
super().__init__(namespacify(cbor2.loads(buf)))
# Store the file pointer for `extend` calls.
self._file = file_
